The present invention relates to a wheel drive unit which is a combination of a wheel support rolling bearing unit, a constant velocity joint unit and a snap ring, and is used for rotatably supporting with respect to a suspension unit, a driven wheel {the front wheel of a FF vehicle (front engine, front drive wheel) the rear wheel of both a FR vehicle (front engine, rear drive wheel) and a RR vehicle (rear engine, rear drive wheel) and all wheels of a 4 WD vehicle (four drive wheel)} supported on an independent suspension type suspension, and for rotationally driving the driven wheel.
In order to rotatably support a vehicle wheel with respect to a suspension unit, various kinds of rolling bearing units for wheel support with an outer ring and inner ring assembled so as to be freely rotatable via rolling elements, are used. Moreover, a wheel support rolling bearing unit for supporting a driven wheel in a suspension of the independent suspension type and rotationally driving the driven wheel, must be combined with a constant velocity joint to smoothly transmit (maintain constant velocity) the rotation of the drive shaft to the abovementioned vehicle wheel, regardless of the relative displacement between a differential gear and the driven wheel and of a steering angle applied to the wheel. FIG. 6 shows a typical bearing unit 3 for wheel drive wherein a rolling bearing unit 1 for wheel support and a constant velocity joint 2 are combined together for this kind of purpose.
This rolling bearing unit for wheel support 1 is found by rotatably supporting a hub 5 and an inner ring 6 on the inner diameter side of an outer ring 4 via a plurality of rolling elements 7. Of these, the outer ring 4 when connectingly secured to a knuckle 9 (refer to FIG. 7 mentioned below) constituting a suspension unit by means of a first flange 8 provided on the outer peripheral face thereof, does not rotate even at the time of use. Furthermore, a double row of outer ring raceways 10 is provided on the inner peripheral face of the outer ring 4, and the hub 5 and the inner ring 6 are rotatably supported on the inner diameter side, concentric with the outer ring 4.
Of these, the hub 5 is provided with a second flange 11 for supporting the wheel, on a portion near an outer end (the end which is on the outside in a widthwise direction of the vehicle when fitted to the vehicle, namely, the left hand side of each figure including FIG. 6) of the outer peripheral face. Furthermore, the first inner ring raceway 12 is formed on a central portion of the outer peripheral face of the hub 5. Similarly, an inner ring 6 with a second inner ring raceway 14 formed on an outer peripheral face thereof, is externally secured to a small diameter step 13 formed on an inner end (the end which is on the middle side in a widthwise direction of the vehicle when fitted to the vehicle, namely the right hand side of each figure). Moreover, in the center portion of the hub 5, a spline bore 15 is provided, so that the hub 5 is formed in a hollow cylindrical shape.
On the other hand, the constant velocity joint 2 has an outer ring 16 for constant velocity joint, an inner ring 17 for constant velocity joint and a spline shaft 18. Of these, the outer ring 16 for constant velocity joint and the spline shaft 18 constitute a drive member 19. That is, the spline shaft 18 is provided on an outer end of the drive member 19 and is freely engaged with the spline bore 15, and the outer ring 16 for constant velocity joint is provided on an inner end of the drive member 19. At a plurality of places on the inner peripheral face of the outer ring 16 for constant velocity joint around the circumferential direction are respectively formed outside engaging grooves 20 at right angles to the circumferential direction. Moreover, regarding the inner ring 17 for constant velocity joint, a second spline bore 21 is formed at right angles to the circumferential direction in a center portion, and on the outer peripheral face, inside engaging grooves 22 are formed at right angles to the circumferential direction in portions coinciding with the outside engaging grooves 20. Moreover, balls 23 are provided between each of the inside engaging grooves 22 and each of the outside engaging grooves 20 so as to be freely rotatable along each engaging groove 22 and 20, with the balls 23 being retained in cage 24. At a part of the inner peripheral face of the outer ring 16 for constant velocity joint, portions between pairs of circumferentially adjacent outside engaging grooves 20 constitute cage guide faces 25. Each cage guide face 25 is positioned on a single spherical surface with a displacement center of the constant velocity joint 2 as the center thereof. Regarding the shape of the constituent elements of this kind of constant velocity joint 2, this is similar to the case of a well known Rzeppa type or Birfield type constant velocity joint, and since this has no relevance to the gist of the present invention, detailed description is omitted.
In combining the above described constant velocity joint 2 and the wheel support rolling bearing unit 1, the spline shaft 18 is inserted into the spline bore 15 of the hub 5 from the inner side towards the outer side. Then a nut 27 is screwed onto an external thread portion 26 provided on an outer end portion of the spline shaft 18 projecting from an outer end face of the hub 5, and then by tightening, these are connected and secured together. In this condition, since the inner end face of the inner ring 6 is abutted against the outer end face of the outer ring 16 for constant velocity joint, there is no displacement of the inner ring 6 in a direction to come off from the small diameter step portion 13. At the same time, an appropriate pre-load is applied to each of the rolling elements 7.
Moreover, when fitted to the suspension unit of a vehicle, a male spline portion 29 provided on an outer end of a drive shaft 28 is spline engaged with a second spline bore 21 provided in a central portion on the inner ring 17 for constant velocity joint. Then, a snap ring 37 which is stoppingly engaged in an engaging groove 30 formed around the whole periphery in the outer peripheral face at the outer end of the male spline portion 29, is engaged with an engaging step portion 32 formed in an opening rim at the outer end of the second spline bore 21, thereby preventing the male spline portion 29 from coming out from the second spline bore 21. Furthermore, the inner end of the drive shaft 28 is connectingly secured to the center of a trunnion 34 (refer to FIG. 1 showing a first example of an embodiment of the present invention) of a tripod type constant velocity joint 33 provided on an output shaft of a differential gear.
Regarding the first example of the abovementioned conventional construction shown in FIG. 6, the weight is increased because the rolling bearing unit 1 for wheel support and the constant velocity joint 2 are connectingly secured based on the threaded and tightened engagement between the male thread portion 29 and the nut 27. That is, providing the external thread portion 26 on the spline shaft 18 of the constant velocity joint 2 requires lengthening of the spline shaft 18, and the nut 27 also becomes necessary. Therefore the axial dimension and the weight of the bearing unit 3 for wheel drive are increased by the male thread portion 26 and the nut 27.
To address this problem, in the specification of U.S. Pat. No. 4,881,842 is disclosed as shown in FIG. 7, a bearing unit 3a for wheel drive that enables shortening of the axial dimension and a reduction in weight, by connectingly securing the rolling bearing unit for wheel support and the constant velocity joint by a relatively simple construction. Also in the case of the secured example of the conventional construction shown in FIG. 7, a hub 5 is rotatably supported on the inside of an outer ring 4 secured to a knuckle 9, by rolling elements 7 arranged in a double row. Moreover, a spline shaft 18 of a drive member 19a is spline engaged with a spline bore 15 formed in a central portion of the hub 5. An engaging portion 35 is formed in an outer end surface of the spline shaft 18 for engaging with a tool for drawing the spline shaft 18 into the spline bore 15. Furthermore, the spline shaft 18 is prevented from coming off from the hub 5 by a snap ring 31 which is stoppingly engaged in an engaging groove 36 formed in an outer peripheral face of the spline shaft 18 at a portion close to its tip end (outer end). In this condition, an resilient ring 34 is resiliently compressed between the hub 5 and the outer ring 16 for constant velocity joint of the drive member 19a, thereby effecting play prevention of the spline shaft 18 and the hub 5. In the case of the second example of this kind of conventional construction, to the extent that connection of the rolling bearing unit 1a for wheel support and the constant velocity joint 2a is performed by the snap ring 31, an overall smaller size and lighter weight for the bearing unit 3a for wheel drive is achieved.
Moreover, in Japanese Patent Publication No. Tokukai Hei 10-264655 is disclosed a bearing unit 3b for wheel drive such as shown in FIG. 8. A rolling bearing unit 1b for wheel support constituting the bearing unit 3b for wheel drive has a hollow hub 5a such that a constant velocity joint outer ring 16a constituting a constant velocity joint 2b corresponding to the drive member described in the claims, is connected to an inner end of the hollow hub 5a via a spacer 38. Of the inner and outer peripheral surfaces of this spacer 38 formed in a short cylindrical shape, an inner diameter side female spline portion 39 is formed on the inner peripheral surface of the spacer 38, and an outer diameter side male spline portion 40 corresponding to the first spline portion described in the claims is formed on the outer peripheral surface of the spacer 38. This spacer 38 is assembled on an outer peripheral face at the inner end of the hub 5a, with an inner diameter side male spline portion 41 formed on an outer peripheral surface thereof in spline engagement without play with the inner diameter side female spline portion 39. Then, in this condition, an inner end face of the spacer 38 is clamped by a crimped portion 42 formed on the inner end of the hub 5a, so that the spacer 38 is secured without play to the inner end of the hub 5a. Moreover, in Japanese Patent Publication No. Tokukai Hei 10-264655 is also disclosed a construction where the inner ring and the spacer are formed as one body.
On the other hand, an outer diameter side female spline portion 43 corresponding to the second spline portion described in the claims, which is formed on the inner peripheral face at the outer end of the outer ring 16a for constant velocity joint is spline engaged with an outer diameter side male spline portion 40. That is, the outer diameter side female spline portion 43 is formed in an inner peripheral face at the outer end of the outer ring 16a for constant velocity joint. Moreover, as mentioned above, the outer diameter side female spline portion 43 is spline engaged with the outer diameter side male spline portion 40 formed on the outer peripheral surface of the spacer 38.
A snap ring 31a spans between the outer diameter side female spline portion 43 and the outer diameter side male spline portion 40 which are spline engaged with each other as described above, so that the outer ring 16a for constant velocity joint cannot separate from the spacer 38. That is, the snap ring 31a is formed in a semi-circle annular shape and made to span between an inside engaging groove 44 corresponding to a first connecting portion described in the claims, which is formed around the whole periphery in the outer peripheral surface of the spacer 38, and an outside engaging groove 45 corresponding to a second connecting portion described in the claims, which is formed around the whole periphery in the inner peripheral surface at the outer end of the outer ring 16a for constant velocity joint. Hence the outer ring 16a for constant velocity joint and the spacer 38 cannot be displaced from each other in the axial direction.
The construction of the connecting portion of the hub 5a and the outer ring 16a for constant velocity joint is as mentioned above, and this hub 5a is rotatably supported on the inner diameter side of the outer ring 4 by a double row angular type ball bearing. An inner ring 6 constituting this ball bearing is clampingly secured between an outer end face of the spacer 38 and a step face 46 existing on the outer end of a small diameter step portion 13 formed on an outer peripheral face at the central portion of the hub 5a. 
In the case of the bearing unit 3b for wheel drive described in Japanese Patent Publication No. Tokukai Hei 10-264605 as described above, the spline shaft 18 can be omitted from the second example of the conventional construction shown in FIG. 7, and to that extent, the cost and weight can be further reduced.
In the case of the second and third examples of the conventional construction shown in FIG. 7 and FIG. 8, in comparison to the case of the first example of the conventional construction shown in FIG. 6, a reduction of cost and weight is effected. However, in order to sufficiently maintain the durability of each constituent element, it is necessary to optimize the properties of each constituent element. That is, at the time of using the wheel drive unit, various kinds of stress such as, force in the compression direction, force of the bending direction, and force in the tension direction and the like, are applied to each constituent element of the rolling bearing unit for wheel support and the constant velocity joint which are combined together to form the wheel drive unit. However with regards to this point, heretofore no consideration has been given to make the properties of each constituent element optimum to deal with these stresses.
The present invention was invented taking this situation into consideration, in order to optimize the properties of each constituent element so as to ensure durability of the wheel drive unit.
The present invention provides a wheel drive unit comprising a rolling bearing unit for vehicle wheel, a constant velocity joint unit and a snap ring, the constant velocity joint unit comprising a first constant velocity joint having an output portion and a input portion connected to an output portion of a differential gear, a transmission shaft having an output end and an input end connected to the output portion of the first constant velocity joint, and a second constant velocity joint having an output portion and an input portion connected to the output end of the transmission shaft, the rolling bearing unit for vehicle wheel comprising an outer ring having an inner peripheral surface formed with outer ring raceways and being not rotatable during use, a hollow hub having an outer peripheral surface formed with an flange for supporting a vehicle wheel near the outer end thereof, with a first inner ring raceway at the middle portion thereof, and with a small diameter stepped portion formed near the inner end thereof, an inner ring having an outer peripheral surface formed with a second inner ring raceway and fitted onto the small diameter stepped portion of the hub, the hub having the inner ring from coming out of the smaller diameter portion, a plurality of rolling members rotatably provided between each of the outer ring raceways and the first and second inner ring raceways, and a first spline portion provided on a peripheral surface portion of the hub or a member securely connected to the hub, the second constant velocity joint comprising a drive member having a peripheral surface at the outer end thereof formed with a second spline portion in spline engagement relation with the first spline portion, and an outer ring for constant velocity joint at the inner end thereof to constitute the second constant velocity joint, a first engagement portion provided on a peripheral surface portion of the hub or a member securely connected to the hub, a second engagement portion provided on the peripheral surface at the outer end of the drive member, the snap ring spanned between the first engagement portion and the second engagement portion to prevent disengagement between the first spline section and second spline section, the smaller diameter stepped portion of the hub having a stepped face portion at the innermost end thereof, the inner ring having an inner end face abutted to the stepped surface portion, on the outer peripheral surface of the hub, at least the first inner ring raceway and the stepped surface portion being quench-hardened, on the inner peripheral surface of the hub, at least a portion located on the inner diameter side of the quench-hardened stepped surface portion, and the crimped portion being not quench-hardened, at least one of the hub or the member securely connected to the hub and the drive member having a peripheral portion being formed with an engagement groove for the first and second engagement portions and not quench-hardened, the outer ring for constant velocity joint being formed with outside engagement groove portions on the inner peripheral surface thereof with cage guide portions each existing between a circumferentially adjacent pair of the cage guide portions, and on the inner peripheral surface of the outer ring for constant velocity joint, at least the outside engagement groove portions and the cage guide portions being quench-hardened.